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I'm trying to use an LM1117 linear voltage regulator to convert to 3.3v (input voltage will be 9 or 5 volts; not yet decided). The datasheet suggests using 10uF tantalum capacitors on the input and output. While I could just go with the suggestion, I find most of the tantalum capacitors that are available are considerably more expensive than other capacitors, and in a SMT form factor (I'd prefer through-hole). I'm thinking of using aluminum or ceramic caps instead.

This excerpt from the datasheet specifies what ESR ranges are acceptable:

8.2.2.1.3 Output Capacitor
The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both minimum amount of capacitance and equivalent series resistance (ESR). The minimum output capacitance required by the LM1117 is 10 µF, if a tantalum capacitor is used. Any increase of the output capacitance will merely improve the loop stability and transient response. The ESR of the output capacitor should range between 0.3 Ω to 22 Ω. In the case of the adjustable regulator, when the CADJ is used, a larger output capacitance (22-µF tantalum) is required.

As I understand it, ceramic caps have a very low ESR. Could I just add a 0.3 Ω to 22 Ω resistor in series with a ceramic capacitor? I've read what ESR is and isn't, and I don't understand it one bit, so right now it's just a range of numbers I'm trying to match. However, ceramic caps tend to have smaller capacitance, so it might not be as easy to get 10uF ceramic caps.

If I recall correctly, aluminum caps have a much higher ESR (though exactly what "higher" means I'm not sure). Additionally, it can be difficult to get a datasheet for a capacitor simply because they're such common components (I'm in China and purchasing online, but don't read Chinese). So I'm not sure how to make sure I'm within the ESR range specified.

Finally I should note that I don't have any of these components yet, but I'll probably get an assortment of capacitors when I do order.

So my question: What capacitors can be used in this situation, and with what value resistors (if any)? Note that I'm interested in the theory, not just this single situation, but not being experienced with capacitors some specific examples could be helpful.

Related: ESR and CSR of Capacitor

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    \$\begingroup\$ "because they're such common components" no, you don't get a datasheet for them because they are cheap common crap. For every reputable cap, there is a more or less useful datasheet \$\endgroup\$
    – PlasmaHH
    Commented Sep 20, 2016 at 15:38
  • \$\begingroup\$ If you prefer PTH, why not just go with electrolytic then? Your low ESR caps should be close to your target circuitry, by then traces/leads will probably be long enough so that they won't de-stabilize the regulator. \$\endgroup\$
    – Wesley Lee
    Commented Sep 20, 2016 at 15:41
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    \$\begingroup\$ Yes, you can just add a series resistor with a ceramic cap. Did you calculate the power dissipation? It is (Vin-Vout) * Iload. With 9V in, the power dissipation may be pretty high. You might need to use a DC-DC converter. \$\endgroup\$
    – user57037
    Commented Sep 20, 2016 at 15:52
  • \$\begingroup\$ @PlasmaHH Yeah, that was poorly worded on my part. Finding the datasheet can be difficult if the seller doesn't list it and there's no part number. Or will good capacitors always have a part number you can look up? \$\endgroup\$
    – Nateowami
    Commented Sep 21, 2016 at 1:21
  • \$\begingroup\$ @mkeith Hmm... I estimated it but it looks like it's going to be higher than I was thinking. It should be 170mA for "typical use," but in reality it should be less most of the time. 170mA*(9v-3.3v)=0.969 W. That's pretty high. The datasheet has graphs for maximum power dissipation at different ambient temperatures, but doesn't list the TO-220 package. My 5v source would be from an Arduino, which can only provide 200mA (I fear I may spike over that limit). But maybe I should run the whole thing from a 5v power supply and bypass the Arduino's 5v regulator. I'm trying to power an ESP8266. \$\endgroup\$
    – Nateowami
    Commented Sep 21, 2016 at 2:55

3 Answers 3

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Yes, you can simply put a resistor in series with a ceramic capacitor. The lower the better from the point of view of bypassing, so I would aim at 0.5 to 1 ohm. If you have lots of space, the electrolytic is fine (in fact you can parallel the two), and they are cheap. There are low ESR electrolytics and ones that are not-so-low, read the datasheet. If no datasheet, no buy.

You should be able to read the numbers on the datasheets even if some of it is in a foreign language. If you're going to the market and picking shiny parts off of vendors displays without looking at datasheets you will get bitten. I've always been able to get answers to questions such as the load capacitance of a crystal without being the most amazing linguist around.

The x1117-3.3 is extremely cheap and very available in China so I don't see any reason not to use it. If you don't need the power dissipation there are better choices in SOT-23.

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  • \$\begingroup\$ If no datasheet, no buy. Should I take this as a general rule, or only when concerned with ESR (or other particulars)? I was thinking of getting a whole set of different values, but maybe that's not a good idea (unlikely to specify a datasheet for each one). From what I've found, usually if there is a datasheet, it's in English (or more often I just search for a datasheet by the part number). \$\endgroup\$
    – Nateowami
    Commented Sep 21, 2016 at 1:17
  • \$\begingroup\$ @Nateowami Usually there is a datasheet (or set of datasheets) to cover an entire range of parts. For example, an entire series of electrolytic caps from one maker. It can be fairly challenging to find some datasheets from domestic Chinese makers. \$\endgroup\$
    – Spehro Pefhany
    Commented Sep 21, 2016 at 1:19
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Yes, you can add deliberate resistance to a low ESR cap to replace a tantalum cap in this case. However, a much better answer is to use a more modern regulator that is stable with a 0 ESR output cap. A LM1117 makes no sense here, since you don't need really low dropout. Even then there are plenty of modern LDOs available that are 0-ESR stable.

Use the right linear regulator, and this problem goes away.

If you are really going to use 9 V in, then even a 7805 would be fine here. They are very robust and tolerant of significant capacitance on their output without any resistance in series. After the next asteroid impact, the only thing left will be cockroaches and 7805 regulators.

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    \$\begingroup\$ Oh dang, and I was stockpiling 555s for after the bang... \$\endgroup\$
    – PlasmaHH
    Commented Sep 20, 2016 at 18:00
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    \$\begingroup\$ @PlasmaHH We speak not of those. For some reason the mere mention of 555s seems to make pros twitch. \$\endgroup\$
    – Spehro Pefhany
    Commented Sep 20, 2016 at 22:17
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    \$\begingroup\$ even a 7805 would be fine here. Pardon me if I'm wrong, but isn't the 7805 5v, not 3.3v? I was planning on using something from the 78** line, but they don't appear to go below 5v (at least I couldn't find any that did). Searching for 3.3v regulators the LM1117 came up, but as this is new ground for me so I'm not sure what to use. Assuming the 78** doesn't have 3.3v, what other regulators should I be looking at? \$\endgroup\$
    – Nateowami
    Commented Sep 21, 2016 at 3:01
  • \$\begingroup\$ Shopping questions are off-topic. HOWEVER, someone recently posted a link to a Murata DC-DC converter that is a drop-in replacement for the 7805 and similar regulators. They have a 3.3V version. These are buck converters, so the power dissipation will be much lower than your calculation above. \$\endgroup\$
    – user57037
    Commented Sep 21, 2016 at 3:10
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    \$\begingroup\$ @Nate: You are right, I forgot you are trying to make 3.3 V, not 5 V. A 78xx specifically may not be a fit, but there are many many linear regulators out there. Finding one that is stable with 0 ESR output cap is not hard. Most newer ones are like that. \$\endgroup\$
    – Olin Lathrop
    Commented Sep 21, 2016 at 10:41
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Increasing capacitance value gives better transient response and allows use of a lower ESR capacitor while keeping the regulator stable.

For this kind of regulator, aluminium electrolytics work fine unless you use the low-ESR kind.

I usually use no-name 105°C 100µF 25V caps which cost 4 cents a piece. They have an ESR around 0.5 ohms.

Basically, for this 3V3 regulator, grab an aluminium electrolytic through-hole cap with the following charatceristics:

  • small and convenient package (6.3mm diameter)
  • largest capacitance value you can find for 6.3V rating (should be 200-400 µF)
  • not specified for low ESR or low-Z
  • brand name or 105°C if you care about service life.

It'll work.

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